A typical example of containers of this kind is a syringe, commonly called "prefilled syringe," which can be loaded with two different substances placed in different compartments, and allows the substances to mix through a bypass under the movement of the gaskets effected by a plunger slidably inserted in one end of the tubular body.
More particularly, in the case of a syringe a liquid medicinal substance is filled in one of the compartments, and then it is freeze-dried into a powdery state. The other substance is a vehicle placed in the other compartment, which is used to dissolve or disperse the powdery medicinal substance. In this way a prefilled syringe is finished. In most cases, the vehicle is sterilized by steam after the tubular body is sealed. The problem arises that the steam serialization is likely to denature the prefilled medicinal substance. To avoid this problem, the medicinal substance is placed after the steam sterilization is completed. This method causes another problem in that moisture from the steam is likely to stay on the gasket and impregnate the gasket or penetrate therethrough, thereby spoiling the desiccated medicinal substance with the moisture.
In order to solve the problem of moisture, there is a proposal for heating the syringe at 100.degree. C. or more for hours so as to dry the moisture, but this high temperature is likely to spoil the vehicle.
In conclusion, the problems of moisture arise from the single structure of the gasket which separates the space into the two compartments.
In order to solve the problems, there is an improved prefilled syringe which is disclosed in EPC Publication No. 0 568 321 A 2. The syringe is provided with a double-structure gasket used for separating the interior space of the tubular body into a front compartment and a rear compartment in a sealing manner. The double-structure gasket refers to a gasket consisting of two parts which are slidable independently of each other. For explanation's sake, the two gasket parts will be referred to as a "front part" and a "rear part".
Because of the presence of a possible gap between the two parts, even after steam sterilization is finished, moisture likely to remain in and on the rear part is prevented from reaching the front part and wetting it.
Each gasket includes annular ribs sealingly engaging the inside wall of the tubular body. Preferably it is additionally provided with bridging ribs extending between the adjacent annular ribs and subdividing a space between the annular ribs. The bridging ribs also sealingly engage the inside wall of the tubular body wherein "sealingly engage" does not always mean that the bridging ribs are compressed against the inside wall of the tubular body.
However, it has been found that it is difficult to ensure that all the annular ribs of the two parts sealingly engage the inside wall of the tubular body at equal pressure. If the contact pressure is not equal, the parts fail to effect an air and liquid thight seal. However, the air and liquid-tight seal and slidability of the parts are mutually contradictory; if the gasket is too tight against the inside wall of the tubular body, it slides on it with difficulty, whereas if it is loose, the gasket can easily slide, but the air and liquid tight seal will be sacrificed.
The compressibility of a gasket can be expressed by the following equation: ##EQU1## where R (mm) is the outside diameter of the annular ribs of the gasket which stands out of the tubular body, being subject to no compression, and r (mm) is the inside diameter of the tubular body.
Japanese Patent Publication (allowed) No. 57-26782 discloses a syringe having a gasket encased therein which is superior in slidability and air and liquid-tight seal. The gasket is made of thermoplastic elastomer whose annular ribs have a compressibility C of 0.6 to 18.3% depending upon the inside diameter of the syringe. In addition, the product of the total contact area S (mm.sup.2) and compressibility (C) is in the range of about 350 to about 900.
Japanese Utility Model Laid-Open Publication No. 3-63344 discloses a gasket having annular ribs at least one of which has a contact area S of 28 mm.sup.2 or more as well as a compressibility C of 1 to 5. The product of the compressibility (C) and the contact area (S) is set so as to fall within the range of about 100 to about 250.
However, problems are likely to arise in these known gaskets when they are used in association with prefilled syringes, because the gaskets are fabricated without taking into consideration any unfavorable influences likely to rise from pre-treatments such as freeze-drying and steam sterilization applied to the syringes or from storage. In encasing the gaskets in prefilled syringes, a likely deformation thereof must be taken into consideration.
The prior art gaskets referred to above are fabricated without taking into consideration possible compression deformation.
More specifically, the Publication No. 57-26782 teaches that for medical purposes the syringe is sterilized with the gasket encased therein using ethylene oxide gas at a temperature of about 60.degree. to about 65.degree. C. for about 6 to 8 hours and recognizes that the compressibility is adequate if it falls in the range of 0.6 to 18.3%. However this prior art literature does not refer to deformation likely to occur when the syringe is freeze-dried or stored for a long time. In addition, it is necessary to make the gasket with a special type of having an optimum C.times.S value so as to improve the slidability.
Another prior art Publication No. 3-63344 does not teach any counteraction against unfavorable influences upon the slidability and liquid-tightness of the syringe when they are freeze-dried and stored over a long time.